There have been developed in the prior art a variety of flowmeters in which a manometer or other device is connected across opposite sides of a flow restrictor.
Mass flow instruments measure the flow rate of a fluid not by directly determining the pressure differential across a restrictor, but by measuring the actual flow of a small portion of the fluid. Such applications require that the flow of the fluid be divided into two or more paths with an exact ratio maintained between the individual path flow rates. Mass flow detection is a means of measuring the weight flow rate of a gas. Controlling mass flow is the same as controlling the flow of molecules since equal volumes of ideal gases, under the same conditions, contain the same number of molecules. In contrast volumetric flow measurement and control must be corrected for local temperature and pressure conditions in order to determine molecular flow.
In a typical mass flow meter, a very small percentage of the flow is diverted into a measuring or sensor section. This percentage may be as small as one part in 40,000 and the measuring section is typically a very thin tubular conduit which is much longer than its diameter so that laminar flow prevails throughout the conduit. During laminar flow of a fluid, the flow rate is directly proportional to pressure drop and inversely proportional to viscosity. In contrast, during turbulent flow the flow rate is proportional to the square root of the pressure drop and largely independent of viscosity. Therefore, in the design of a mass flow instrument it is important to provide conditions that ensure laminar flow in each path.
The bypass assembly or primary fluid path precisely splits the gas flow, directing a sample of qas through the sensor and enabling the laminar flow of the remaining gas through the instrument. The bypass assembly generally comprises a plurality of closely-spaced fluid passageways, each passageway having an effective diameter sufficiently small for assuring laminar fluid flow. For example, a plurality of fine mesh screen discs are stacked together to obtain a desired pressure drop by defining a plurality of elongate continuous channels bounded by the screen material. Alternatively, screen material has been spirally wound around a mandrel and secured in its wound form to form channels between the spiral layers. Laminar flow may also be ensured by using as a flow restrictor one or more juxtaposed discs, each having channels formed from its perimeter to an opening through opposite sides of the disc. The fluid is directed to the perimeter of the disc and is conveyed by the conduits to the opening, thus forming an elongate channel having a sufficiently large length to diameter ratio to assure laminar flow of the fluid.
Mass flow instruments accurately and reliably measure and control mass flow rates of gases from below 5 standard cubic centimeters per minute (SCCM) to about 500 standard liters per minute (slm) without the need for pressure or temperature corrections. The flow-measuring section or sensor assembly may comprise a tube or conduit externally wound with two heated resistance thermometers to measure the gas flow. For example, in a particular mass flow instrument, a bridge circuit senses the temperature differential and develops a linear output signal of from 0 to about 5 vdc proportional to the gas flow rate over the calibrated range. In a mass flow controller, the signal is compared to a command voltage from a potentiometer or voltage source. This comparison generates an error signal that alters the valve opening, thereby changing the flow rate until a set point is reached. A feedback circuit provides dynamic compensation for optimum stability and response. In order to maintain optimal calibration of the mass flow instrument, the flow rate through the sensor conduit must be maintained within the range of laminar flow in order to maintain a linear output signal over the full-scale flow rate; e.g., 0 to 5 vdc with regard to the sensor hereinbefore described.
The bypass assemblies hereinbefore described provide laminar flow in the primary fluid path of the mass flow instrument and allow the accurate and reliable measure and control of the mass flow rates by such instruments within certain ranges. However, as the output of the sensor assembly is dependent upon the rate of fluid flow therethrough and thus the pressure drop across the bypass flow restrictor, the flow ratio, i.e. the amount of fluid passed by the bypass section in relation to the measuring section, can be changed only by replacing the flow restrictor in the bypass assembly to allow a greater or lesser volume of fluid across a pressure drop which maintains the optimal sensor flow. That is, once the instrument is calibrated to respond to a given sensor flow range, the total flow through the instrument cannot be substantially varied by changing the input pressure of the fluid, as the pressure variations will cause the sensor flow to extend beyond the calibrated limits, i.e., beyond the laminar flow range of the sensor tube.
Thus, in the past the flow ratio has been modified by the substitution of replaceable laminar flow-forming elements having varied channel or passage sizes which allow a different primary flow rate while retaining a substantially similar pressure drop. However, as such replaceable elements require disassembly of the bypass flow restrictor, it has been a desideratum to provide an adjustable laminar flow bypass assembly which enables the continuous adjustment of flow ratios without requiring the disassembly of the bypass flow restrictor and the substitution of predetermined conduits therethrough.
The present invention provides a bypass assembly which is continuously adjustable to provide a wide range of flow volumes while allowing the maintenance of a constant pressure drop across the bypass assembly to ensure a constant and optimal flow through the sensor tube. This is accomplished by providing a frusto-conical flow restrictor having a perimeter surface essentially parallel to the surface of a frusto-conical bore, and capable of defining an annular conduit therebetween having an annulus being of a thickness to width ratio to provide laminar flow, the frusto-conical portion being axially and selectively movable within the frusto-conical bore to provide annuli having variable thicknesses. Several related embodiments are disclosed, each providing the described advantages. The adjustable laminar flow bypass assembly of the invention may be combined with an elongate laminar flow conduit, serving as a measuring section, to constitute a linear flowmeter. Such a meter includes a housing having a fluid inlet and a fluid outlet, the housing defining a fluid path between the inlet and outlet. The adjustable flow element is disposed in this fluid path to define an annular conduit having dimensions appropriate for the maintenance of laminar flow, in parallel circuit with the measuring section conduit. Means are provided for measuring the rate of flow of fluid through the measuring section conduit, which means are known in the art and constitute no part of the present invention.